Many methods of processing ferrous metal materials (iron and steel), such as crushing, grinding, cutting, polishing, machining, screening and melting ferrous metal parts, produce large amounts of fine ferrous metal particles. It is desirable to recover and reprocess these partides. The ferrous metal has significant value if it can be remelted by a foundry, smelter or steel making operation. It is also environmentally preferable to melt existing metal scrap instead of extracting new metal from the earth as ore.
Unfortunately, particles of ferrous metals will oxidize (rust) rapidly at ambient temperature, particularly in humid weather. When such ferrous particles are added to a hot melting furnace, they are likely to be oxidized so rapidly that they will bum before melting. Additionally, fine, light particles of ferrous metal are often blown out of the melt bath and into the atmosphere by heat convection or by the air blast from a cupola furnace. Finally, the iron in such iron oxide partides that do not bum and are not blown away are nevertheless difficult or inefficient to recover by melting, and often end up being lost into the slag produced as a byproduct of the melting process. Thus, most of the ferrous metal partides produced in ferrous metal processing are so problematic to recycle, that they are regarded as practically non-recyclable. They therefore are often placed in landfills and lost forever. A method of forming these fine ferrous metal particles into compacted masses or briquettes that resist oxidation and can be simply and efficiently re-melted in foundries, smelters or steel making operations, would constitute an important improvement in the current state of the art.
The present invention is directed to ferrous metal masses or briquettes bonded by a glassy binder coating and to a method of making those masses and using them in recycling ferrous metal particles.
The ferrous metal masses (iron and steel) comprise a mixture of at least 80% by weight ferrous metal particles and preferably over 90% by weight ferrous metal particles. They also include an alkali metal silicate. Forming the particles into briquettes makes it possible to conveniently and efficiently recycle the particles by adding them to a ferrous metal melt.
In carrying out the method of the present invention, ferrous metal particles and an alkali metal silicate are mixed together in any convenient manner, and shaped into masses or briquettes, preferably compacted, and caused to cure or harden by gelation and/or dehydration. After curing, the mixture should be at least 80%, and preferably at least 90% by weight of ferrous metal with the balance being an alkali metal silicate and other inorganic impurities, such as silica sand.
The ferrous metal particles are recovered from ferrous metal processing, including crushing, grinding, cutting, polishing, and machining operations. The particles may range in size from fine powders to coarse grains. Particles in the range of about 100 mesh to about 10 mesh are preferred. However, larger and smaller particles can also be utilized. It is desirable to ensure that the ferrous metal particles are kept free from rust prior to being combined with the silicate. Various liquid alkaline alkali metal silicates may be used in the practice of the invention, including sodium silicate, potassium silicate, and lithium silicate. Among these, sodium silicate is preferred. Additionally, one or more silicates may be combined in the mixture. The silicate should be used in the form of an aqueous solution containing from about 20% to 50% by weight solids preferably 35% to 45% by weight solids, and most preferably about 38% by weight solids, with the balance water. Where the ferrous metal particles are provided in a form which already includes water, the alkali metal silicate may be provided, in whole or in part, in dry form so that the already present water supplies some or all of the water in the alkali metal solution.
Curing of the briquettes is believed to proceed by causing the alkali metal silicate solution to form a gel, which contracts and forces the water out of the ferrous metal/silicate mixture, thereby hardening the final product. Thus, during the curing of the briquettes, the moisture in the alkaline metal silicate solution is given up to the atmosphere.
Also, it is preferred that the gelling of the silicate be accelerated by adding an appropriate amount of an accelerant. For example, the following materials can be used as curing accelerants for this purpose: calcium silicate, aluminates such as calcium aluminate, sodium aluminate, magnesium aluminate, potassium aluminate and lithium aluminate; acetic acid esters such as ethylene glycol diacetate, ethylene glycol monoacetate, glycerol triacetate, glycerol diacetate, and glycerol monoacetate; carbonic acid esters such as ethylene carbonate and propylene carbonate; formic acid esters such as methyl formate and ethyl formate; lactic acid esters; adipic add esters; glutaric acid esters and succinic add esters. Injecting carbon dioxide gas into the mixture is another method of accelerating the curing process. Heating is yet another way to accelerate the curing, and may be used either alone or in combination with any of the other accelerants. Indeed, any known method used to cause the curing of the alkali metal silicates solutions may be utilized to harden the ferrous metal particles fines into convenient shapes.
In the absence of the addition of accelerants or the use of heating, the briquettes may be hardened by simply permitting them to dehydrate until hardened.
The mixture of ferrous metal particles and liquid alkaline alkali metal silicate solution is compacted into appropriately sized forms corresponding to the size and shape of the mass desired before allowing the mixture to cure, either slowly at ambient temperature, or more rapidly by adding accelerants as described above. The briquettes may be of any practical size convenient for melting in the particular furnace being used. For example, briquettes of about 2xe2x80x3xc3x972xe2x80x3xc3x972xe2x80x3 up to 10xe2x80x3xc3x9710xe2x80x3xc3x9710xe2x80x3 in size have been made and used in accordance with the invention in induction furnaces and in an Iron foundry cupola. Briquettes less than about 2xe2x80x3xc3x972xe2x80x3xc3x972xe2x80x3 in size are less desirable because of the danger that they will be blown out of the melt bath.
Surprisingly, although one would expect the exposed ferrous metal particles to rust on the surface of the final briquette produced, this does not occur. Rather, the silicate forms a glassy coating on the ferrous metal partides, preventing the briquettes from rusting, even under humid conditions. Furthermore, porosity of the briquette would also be expected which would produce rusting that works its way through the briquette, weakening the briquette and reducing its reuse value. Again, surprisingly, this is not found not to be the case.
The melting point of the metal particles fines is controlled by their chemistry. When the briquettes are added to a hot melting furnace maintained at a temperature sufficiently high to cause melting of the metal particles fines, the entire briquette breaks up, releasing the iron into the melt, while the silicates float up to the surface to become part of the slag. For example, it has been found that many such briquettes will melt at a temperature of at least about 2400xc2x0 F., and up to about 3000xc2x0 F. This occurs with little or no loss of metal due to high temperature oxidation, and certainly no blow out of fine metal particles. Indeed, it is believed that the glassy coating of the ferrous metal particles by the silicate continues to remain in place at the high melting temperatures, until after the ferrous metal particles melt into the furnace bath, whereupon the sodium silicate becomes a part of the slag floating on top of the molten metal.
The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill how to make and use the invention. These examples are not intended to limit the invention or its protection in any way.